The twin explosions that ripped through the crowd near the finish line of the Boston Marathon triggered a massive manhunt that paralyzed a city. Two bombs set off about ten seconds and 100 yards apart signaled the end of one race and the start of another -- to identify and find those responsible. This is how that race unfolded.
MONDAY, APRIL 15
-- Just before 3 p.m., an explosion shatters the cheers on Boylston Street near the finish line of one of Boston's largest and most cherished events. More than 17,000 runners already had crossed the finish line, but thousands more still were headed for the site of the bombing. Ten seconds later, a second explosion shatters windows and bodies. Sirens and screams erupt as rescuers scramble and the crowd panics.
"They just started bringing people in with no limbs," runner Tim Davey of Richmond, Va., said of his view from inside a medical tent that had been set up to care for fatigued runners.
-- The blasts killed three people -- 8-year-old Martin Richard, of Boston; 29-year-old Krystle Campbell, of Medford; and 23-year-old Lu Lingzi, a Boston University graduate student from China -- and injured more than 180 others, but it would be hours before the chaos cleared enough to give authorities a true sense of the casualties. Or even where the bombs had been hidden. If they had been hidden.
-- A citywide shutdown that will become nearly complete by the end of the week begins by early evening. A no-fly zone is created over the bombing sites, major sporting events are canceled, people are urged to stay indoors, SWAT team members with machine guns patrol hospitals. And the world takes notice, beefing up security at nuclear plants, public transit systems and anywhere crowds gather.
-- Is it terrorism? Americans are eager for answers, but when President Barack Obama addresses the nation three hours after the explosion, he stops short of that. "We will find out who did this. We'll find out why they did this," Obama said in his brief statement. "Any responsible individuals, any responsible groups, will feel the full weight of justice."
-- Knowing that thousands of smartphones and cameras were in the crowd, by nightfall authorities officially tap the power of crowd sourcing and put out the call for pictures, videos and tips.
TUESDAY, APRIL 16
-- The day begins with a city -- and a nation -- on edge and without answers. No suspects. No motives. No claims of responsibility. An apartment in nearby Revere was searched overnight, but no details emerge. Copley Plaza -- the typically bustling site of the bombings -- was blocked to vehicles and pedestrians.
-- By noon, Obama inches the nation forward but only barely. Calling the bombings "a heinous and cowardly act," he says they are being investigated as an act of terrorism, but authorities still don't know who is responsible. Later in the day, Homeland Security Secretary Janet Napolitano says the bombings don't appear to be part of a large plot, but security on public transit nationwide -- and around the world -- remains high.
-- A picture of the bombs begins to emerge. Based on debris at the site, investigators determine the bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings. And they were hidden in black backpacks and left on the ground.
-- Pictures of the victims emerge, too. Photos flood social media. There is 8-year-old Martin Richard, smiling and holding a sign that calls for peace and reads, "No more hurting people." And there is 27-year-old Jeff Bauman Jr., being pushed in a wheelchair from the scene of the explosions, bloodied with both legs blown off below the knees.
-- It would be another two days before pictures of the suspects, 19-year-old Dzhokhar A. Tsarnaev and his brother, 26-year-old Tamerlan Tsarnaev, would emerge. But already the younger of the two appears nervous. The owner of an auto body shop near the brothers' Cambridge home later recalled a visit from Dzhokhar on Tuesday.
Gilberto Junior said the usually easygoing teen often stopped by to talk cars and soccer. But on Tuesday, he was biting his nails and trembling. The mechanic told Dzhokhar he hadn't had a chance to work on a Mercedes the teen had dropped off for bumper work. "I don't care. I don't care. I need the car right now," Junior says Dzhokhar told him.
WEDNESDAY, APRIL 17
-- Investigators plow through thousands of tips, scour the rooftops and roads around the blast site, and use sophisticated software to sift through mountains of images and video for patterns or unusual behavior. Obama signs an emergency declaration to send federal aid to Massachusetts.